MIDDLE TO LATE DEVONIAN SKELETAL CONCENTRATIONS FROM TROPICAL CARBONATE-DOMINATED SETTINGS OF NORTH AMERICA: EVALUATING THE EFFECTS OF BIOCLAST INPUT AND BURIAL RATES

Skeletal concentrations, where bioclasts >2mm constitute ≥10% by volume, reflect the interplay among bioclast input (production minus destruction) and burial (net sedimentation) rates. The relative influence of these rates, which controls scales of time averaging, can be teased apart by exploring the stratigraphic, taphonomic, and paleoecologic attributes of skeletal concentrations.

This study compares skeletal concentrations from Middle-Upper Devonian tropical carbonate-dominated strata across distinct subsidence settings. The stratigraphic record of Nevada reflects deposition on a thermally-subsiding passive margin and is up to seven times thicker than coeval strata from Iowa, which reflect deposition on a cratonic interior of equivalent paleolatitude, subtidal facies, and macroinvertebrate fauna.

In both areas, bioclast packing ranges from dispersed to dense, and few skeletal concentrations are both thick (>30cm) and densely packed. Nearly 60% of skeletal concentrations are ≤30cm. The majority of skeletal concentrations are monotypic (composed mostly of brachiopods, but also stromatoporoids, crinoids, and bryozoans), have simple internal stratigraphy, and moderate to high degrees of fragmentation and disarticulation. These results imply that these skeletal concentrations reflect low rates of bioclast production consistent with the low metabolism and fecundity noted among living representatives of Paleozoic fauna.

Although the skeletal concentrations are rarely both thick and densely packed, and few have complex internal stratigraphy, net sediment accumulation rate does exert influence on scales of time averaging. Compared to cratonic Iowa, the subsiding margin of Nevada is more dominated by densely packed, monotypic skeletal concentrations, the majority of which are relatively thin. Iowa preserves a higher proportion of skeletal concentrations with complex internal stratigraphy and mixed states of taphonomic damage. This suggests that skeletal concentrations on the craton are more time averaged than those on the continental margin. These results have implications for Phanerozoic-scale evolutionary trends, given that much of the Paleozoic fossil record comes from carbonate-dominated epeiric seaways and these depositional settings declined through the Phanerozoic.